The present invention relates generally to ablation catheter systems that use electromagnetic energy to ablate internal bodily materials. More particularly, a tunable catheter arrangement particularly suited for use in ablation systems that operate at microwave frequencies is disclosed.
Catheter ablation has recently become an important therapy for certain cardiac arrhythmias. Radio frequency (RF) energy is presently accepted as the preferred ablating energy source. Accordingly, a variety of RF catheters and power supplies are currently available to electrophysiologists. Radio frequency energy has several limitations including the rapid dissipation of energy in surface tissues resulting in shallow "burns" and failure to access deeper arrhythmogenic tissues. Another limitation is the risk of clot formation on the energy emitting electrodes. Such clots have an associated danger of causing potentially lethal strokes in the event that a clot is dislodged from the catheter. For these and other reasons, significant attention has been given recently to alternative ablative energy sources.
A second common ablation approach is the use of high voltage, direct current defibrillator discharges. Direct current ablation has several drawbacks including the need for general anesthesia and explosive discharges that can cause debris or even rupture of certain cardiac organs.
Microwave frequency energy has long been recognized as an effective energy source for heating of biological tissues and has seen use in such hyperthermia applications as cancer treatment and preheating of blood prior to infusions. Accordingly, in view of the drawbacks of the traditional catheter ablation techniques, there has recently been an interest in using microwave energy as an ablation energy source. The advantage of microwave energy is that it is much easier to control and safer than direct current applications and it is capable of generating substantially larger lesions than RF catheters, which greatly simplifies the actual ablation procedures.
In U.S. Pat. No. 4,641,649, Walinsky et al. disclose a medical procedure for the treatment of tachycardia and cardiac disrhythmia which uses microwave frequency electro magnetic energy to ablate selected cardiac tissue. The microwave energy is transmitted over a coaxial transmission line having an antenna at its distal end. A similar procedure is disclosed in Langberg et al's article entitled "Catheter Ablation of the Atrioventricular Junction Using a Helical Microwave Antenna: A Novel Means of Coupling Energy to the Endocardium," PACE, pp. 2105-2113 Vol. 14 (1991). As suggested in the title, the Langberg et al. article proposes the use of a helical microwave antenna at the distal end of the catheter in order to improve the catheter's power delivery characteristics.
In U.S. Pat. Nos. 4,945,912, and 5,246,438, Langberg details particular helical antenna designs to be used for cardiac tissue ablation. In the disclosed design, a distal electrode is directly coupled to the antenna by a peripheral terminal. A bypass capacitor is also coupled to the peripheral terminal in an attempt to ground RF energy before it reaches the distal electrode. Although the arrangement disclosed by Langberg may have many advantages, the antenna is directly coupled to an electrode. Therefore, there is a risk that the grounding will not always be 100% effective and that this device may suffer some of the same limitations of the RF devices such as charring. In the later patent, Langberg recognizes the importance of adjusting the catheter impedance for particular ablation conditions. However, he proposes setting a particular presumed optimal impedance during fabrication. However, this design is not real-time tunable to compensate for the time variation of the impedance over the course of an ablation procedure.
Catheter diameters in cardiac ablation applications are typically restricted to diameters of about 71/2 French (approximately 2.5 mm in diameter). One problem that arises when using the very small diameter transmission lines that are necessitated by such diameter limitations is that the attenuation is quite large over the length of the transmission line. More troublesome is that during use, this attenuation can result in significant heat generation in the transmission line and catheter.
A frequent concern in the management of microwave energy is impedance matching of the various transmission line components. An impedance mismatch will reflect some portion of the incident power resulting in reduced energy transmission and increased losses, typically manifested as heat generation due to line or wave guide attenuation. The effects of these mismatches can be minimized through a process of "impedance matching" with the use of a variety of tuning device configurations and methods. Efficient use of these methods mandates close proximity of the device with the source of the reflected power.
With microwave energy ablation, as with radio frequency ablation, the points of greatest impedance mismatch are located at the tip of the catheter. Further, the impedance on the catheter side of the device (as distinguished from the power supply or energy source) tends to vary a fair amount as the catheter is moved about during use and as tissue properties change during an ablation procedure. For example, the impedance of the catheter to tissue coupling will vary with the location at which the catheter tip is placed in the heart. It will also vary during the course of a typical ablation procedure due to changes in the tissue properties as the target tissue is ablated and heating of the transmission line components. When the impedance changes, an increased percentage of the power is reflected and the catheter's performance is reduced. By way of example, in a representative application wherein the transmission line is approximately one meter long and is a coaxial transmission line having a diameter of 72 thousandths of an inch (1.8 mm), the power output of a well tuned system may only be in the range of 25-30% of the input power. Of course, the power output is likely to improve as the technology develops, but attenuation is always likely to be a significant concern.
To address the attenuation problem, the applicants proposed a tunable catheter arrangement that facilitates impedance matching in the above referenced parent U.S. patent application Ser. No. 08/062,637. The referenced parent application describes novel mechanisms for tuning the catheter system to facilitate on-line impedance matching between the microwave generator side and the catheter side of the system. In one aspect, the application describes a tuning device located remotely in the power supply. Though this location is convenient from a control standpoint and has been used with success, it is inherently somewhat inefficient. Accordingly, the present invention seeks to expand upon the tuning concepts disclosed in the parent application. More specifically, the present application proposes a variety of tuning solutions that compensate for impedance variations in the vicinity of where the variations are generated.